What Are Those Ugly Black Streaks on My Roof?

Expert Contributor

(And How to Remove Them)

by Tom Rafferty, Owner of Rafferty Painting & General Contracting

Although dark spots and ugly black streaks on your roof may look like dirt, mildew, soot or mold, what they really are is algae. The most common type is known as gloeocapsa magma — also known as blue green algae. This algae growth usually begins as small spots and then eventually becomes visible streaks. By the time this algae is noticed by the homeowner, it has usually been on the roof at least two months.

Why Do Shingles Get Algae?

Algae spores are airborne. They can be carried by wind, animals and even on your clothing. Because these spores are airborne, they can quickly spread from rooftop to rooftop in a neighborhood. This is also true for townhomes, condos and apartment complexes.

A very dirty roof BEFORE professional cleaning

Roof algae seems to be more common today than it was 20 years ago. Traditionally, asphalt shingles got their name from being made up of mainly asphalt — an oil-based product — and felt material. To save money, most shingle manufacturers have switched to shingles containing asphalt but mostly made up of fillers such as fiberglass and crushed limestone, which encourages the growth of algae. These algae use the crushed limestone filler as their main food source.

Is This Harmful to the Roof?

Absolutely! This algae feeds off the lime on the roof, causing it to deteriorate. You can reduce the 20-year roof life in half if you don’t maintain it.

The now very clean SAME roof AFTER professional cleaning

How Can I Remove It?

You should never power wash an asphalt roof. If a contractor suggests doing that they do not know what they are doing. A qualified contractor has special low-pressure cleaning machines to ensure no damage to your roof. You must maintain your roof to ensure its lifespan and keep your property looking great.

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